Dinaphthothiophenes (DNTs) are a class of compounds with potential uses in organic semiconductors and the synthesis of asymmetric catalysts. Symmetrical or asymmetrical addition of functional groups to the dinaphthothiophene structure may be desired for steric bulk in binaphthyl catalyst synthesis or tuning the electronic properties of semiconductors or photooxygen precursors. Thus, versatility of functional group addition is a great asset in DNT synthesis. Until now, no versatile and concise methods for the synthesis of asymmetrically substituted dinaphthothiophenes have been reported.
Dinaphthothiophenes are a class of compounds structurally similar to thiophene-based organic semiconductors [1-3] and have shown promise for use in p-type organic semiconductors.[4,5] Dinaphthothiophenes have also been used as precursors to axially chiral 1,1′-binaphthyl catalysts, which play a large role in asymmetric synthesis. [6-8] In addition, dibenzothiophene S-oxide (DBTO) and its derivatives are a class of compounds suggested to release O(3P) upon irradiation with UV light.[9-12] Dinaphthothiophene S-oxides and other fused ring thiophene S-oxides have been investigated for their potential to release atomic oxygen during irradiation at longer wavelengths.[13] Despite their potential applications, few efficient ways to prepare asymmetrically substituted dinaphthothiophenes have been reported.

Dinaphtho[2,1-b:1′,2′-d]thiophene “DNT-2112,” dinaphtho[1,2-b;1′,2′-d]thiophene “DNT-1212,” and dinaphtho[1,2-b:2′,1′-d]thiophene “DNT-1221” are three types of DNTs whose syntheses have previously been reported. DNT-2112 has been synthesized from the Newman-Kwart rearrangement of dithiocarbamates by heating the dimethylthiocarbamate of binaphthol neat at 285-310° C. to give the DNT-2112 in 20-40% yield [8,14,15], from dinaphthyl sulfide using an iodine-catalyzed photocyclization in 85% yield [16], and from cyclization of alkynes by heating ethynyl sulfides in benzene at 200° C. in a cascade cycloaromatization with 10% yield.[17] Rabindran and Tilak performed the condensation of 2-bromo-1-tetralone with 2-naphthalenethiol or 1-naphthalenethiol, followed by cyclization with P2O5 in phosphoric acid and dehydrogenation with selenium, giving DNT-2112 and DNT-1212 in 78% and 76% overall yield, respectively.[18] Morrison and Musgrave used the condensation of thiophene with 1,2-diphenylethanone to give (E,E)-2,5-bis(α-phenylstyryl)thiophene.[19] The phenyl-substituted distyrylthiophene was then photocyclized with iodine to give the diphenyl substituted DNT-2112 in 10% yield. The drawbacks of these preparations of DNT-2112 are that they give low overall yields and take two to four steps to prepare.
DNT-1221 derivatives have been made from the reaction of naphthalene-1-sulfonic acid dimethylamide with n-butyllithium and S8 in 29-37% yield. [20] In addition, both DNT-2112 and DNT-1221 have been synthesized from dinaphthyl sulfides using a potassium tert-butoxide or n-butyllithium induced cyclodehydrogenation in 18-31% yield. [21,22] These syntheses of DNT-1221 suffer from low overall yields. While the final cyclization reactions to synthesize all three varieties of DNTs usually require only one step, anywhere from one to six steps may be required to synthesize the precursors needed for the cyclization reaction from commercially available materials. In addition, the methods requiring fewer steps to reach the cyclization precursor tend to have a more limited scope of synthesis. For example, the method of Morrison and Musgrave, which is the sole method to require only one step to achieve the cyclization precursor, lacks the ability to generate unsubstituted DNTs and has only been used to make diphenyl substituted DNT-2112.[19].
All three classes of DNTs have been synthesized from the flash vacuum pyrolysis of diethynyl/dichlorovinyl-diphenylthiophenes in 7-89% yield.[23] The flash pyrolysis method has some capability for functionalization of the DNT structure. However, it is only able to functionalize symmetrically, which limits the potential for the tuning of the electronic properties of DNTs by tuning functional groups. Furthermore, Tedjamulia et al. prepared all three classes of DNTs from formyl-benzonaphthothiophenes.[24] A Horner-Wadsworth-Emmons reaction was used to add a styrene unit, followed by a iodine-catalyzed photocyclization which gave yields of 45-76%. The synthetic route created here has potential for use in asymmetric DNT substitution, since the DNT core structure is assembled one half at a time; however, controlling the final position of the functional group would be difficult due to the variability of products of the final photocyclization step. This method also has the disadvantage of requiring three to six steps to reach the photocyclization precursor.

While DNTs have previously been asymmetrically functionalized, this has typically been done after the synthesis of the DNT structure. Cho et al. have taken DNT-2112, and functionalized the 6-position of DNT-2112 by use of tert-butyllithium and iodine.[8] In addition, n-butyllithium and DMF have been used to add an aldehyde group, also at the 6-position. [25,26] This indicates that the 6 (and 8) positions of DNT-2112 are selectively deprotonated by bases such as n-butyllithium, leaving the other positions on the dinaphthothiophene rings unable to be so similarly substituted.
In short, a variety of synthetic routes have previously been reported to produce unfunctionalized and a few functionalized DNTs. However, none of these methods begin with the thiophene ring and therefore require a greater number of steps to reach the dinaphthothiophene structure. Furthermore, these methods do not provide a simple way to asymmetrically incorporate functional groups onto the DNT structure.